IRS vs agency

IRS and agency both are nouns.

Word	Noun	Adjective	Verb	Adverb
IRS	Yes	No	No	No
agency	Yes	No	No	No

As nouns, agency is a hypernym of IRS; that is, agency is a word with a broader meaning than IRS:

Other hypernyms of IRS include authority, bureau, federal agency, government agency, office.

IRS (noun)	agency (noun)
the bureau of the Treasury Department responsible for tax collections	to have the ability to produce a particular effect or achieve an end
	an administrative unit of government
	a business or organization that provides a particular service, especially the mediation of transactions between two parties
	the state of serving as an official and authorized delegate or agent
	the state of being in action or exerting power